Field of the Invention
The present invention relates to an image forming apparatus configured to form an image on a fed sheet.
Description of the Related Art
Hitherto, an image forming apparatus such as a copying machine or a printer includes a sheet feeding cassette in an apparatus main body so that copying or printing can be carried out for a sheet continuously. A size and a type of the sheet to be used greatly depend on an environment in which a user uses the image forming apparatus. To address such a requirement, the image forming apparatus is provided with a plurality of sheet feeding cassettes. Even when the image forming apparatus includes a plurality of sheet feeding cassettes, there may arise a demand to execute the copying or printing for a sheet different from the sheet set on each of the sheet feeding cassettes. On the assumption that there may arise such a demand, the image forming apparatus is configured as follows. Specifically, the image forming apparatus also includes a manual feed tray besides the sheet feeding cassettes so that a desired sheet can be set with ease.
In order to feed and convey the sheet set on the manual feed tray and form an image in an appropriate position, the size of the sheet needs to be known, and it is necessary for the user to input the size of the sheet through an operation portion. In this case, each time the user sets a sheet on the manual feed tray, the user needs to set the sheet size through the operation portion, and hence there has been a fear for a decrease in usability. In contrast, there has been proposed such a configuration that a plurality of sensors is arranged on the manual feed tray in a direction of conveying the sheet (Japanese Patent Application Laid-Open No. 2004-231410). A length of the sheet placed on the manual feed tray in the conveyance direction is detected by the plurality of sensors. Based on a value acquired from a volume sensor mounted to a movable guide on the manual feed tray, a length of the sheet in a direction orthogonal to the conveyance direction is detected. Based on the detection results, the image forming apparatus automatically detects the sheet size. In the image forming apparatus configured to automatically detect the sheet size, in order to further enhance operability of the user, the sheet size is determined without displaying a screen for checking the automatically detected sheet size on the operation portion.
The related-art image forming apparatus has the following problems. When a sensor arranged in the conveyance direction of the sheet fails, and a sheet to be subjected to copying or printing is set on the manual feed tray, the sheet size of the sheet set on the manual feed tray is erroneously detected. For example, a sensor arranged in the conveyance direction of the sheet for detecting a sheet having the size of an A3 length (420 mm) may fail and be stuck in an on state. In this case, even when the user sets a sheet having an A4 size (210 mm) on the manual feed tray, the image forming apparatus determines that the sheet size is the A3 length. As a result, the image forming is carried out so as to be suitable for the size of the A3 length. However, the fed sheet is the sheet having the A4 length, and hence the size of a formed image is larger than the size of the fed sheet. As a result, remaining toner that has not been transferred onto the sheet is adhered to members of an image forming portion, which causes an image failure such as marking back during image forming processing on the subsequent sheet.
The sensor arranged in the conveyance direction of the sheet may fail and be stuck in an off state. In this case, even when the user sets a sheet having the A3 size on the manual feed tray, the image forming apparatus determines that the sheet size is the A4 length. As a result, the sheet size determined by the image forming apparatus is smaller in length than the sheet size of the fed sheet in the conveyance direction, and it may be determined that a paper jam occurs.